1. Field of the Invention
The present invention relates to a positioning control system, which includes a tracking servo contro l and a focusing servo control utilizing an optical beam in a magneto-optical disk device, an optical disk device, an optical card, or the like, and which enables a desired track position, etc., to be accurately determined.
More specifically, the present invention relates to a positioning control system including a servo-error signal generating circuit, which can generate servo-error signal in accordance with a detection current output from at least one photo-detector for detecting the return optical beam reflected from a magneto-optical disk device, etc., so as to accurately irradiate a given optical beam to the desired track position(just focus position), etc., for recording/reproducing operations.
2. Description of the Related Art
In general, in a magneto-optical disk device, an optical disk device(sometimes including a magneto-optical disk device), or the like, recording/reproducing operations corresponding to write/read operations can be easily performed by utilizing an optical beam without necessity for complex mechanical elements such as a magnetic head, unlike a magnetic disk drive, a magnetic tape apparatus, or the like. Therefore, the former magneto-optical disk device, etc., can have a larger storage capacity than the latter magnetic disk device, etc. However, the magneto-optical disk device, etc., is likely to have difficulty in positioning a given optical beam to the desired track position, etc.
In regard to the positioning control system of such an optical beam, two kinds of control operations are performed: one is a tracking servo control which enables the track following operations on an optically recording medium such as a magneto-optical disk to be performed; and the other is a focusing servo control which enables a focusing position of the optical beam on the optically recording medium to be determined.
These control operations are executed in the following sequences:
First, the return optical beam, which is reflected from the optically recording medium, is detected by a plurality of photo-detectors in two-divisional configurations or in four-divisional configurations; and
Second, a tracking servo-error signal used for tracking servo control and a focusing servo-error signal used for focusing servo control are simultaneously taken out by utilizing the respective photo-detectors.
In this case, to assure the servo control operations in the magneto-optical disk device, etc., by utilizing an optical beam only, it is necessary to provide a servo-error signal generating circuit which generates each of the above-mentioned servo-error signals adjusted to have a constant amplitude. Such an adjustment of each of the servo-error signals is usually achieved by executing an automatic gain control(AGC) for the servo-error signals on the basis of the detection current output from each of the photo-detectors. Such a servo-error signal generating circuit including AGC circuit portion is generally constituted by a lot of electronic components, such as transistors and resistors. Therefore, a positioning control system having the servo-error signal generating circuit in a magneto-optical disk device, is likely to be large in size and expensive. To avoid this disadvantage, these electronic components are desired to be incorporated into an integrated circuit (IC), which can provide a desired circuit with a small size and in relatively low cost for fabrication.
Here, to enable a problem regarding a conventional positioning control system utilizing an optical beam to be understood more clearly, a concrete configuration of a positioning control system according to a prior art will be described with reference to the related drawings of FIGS. 1(A), 1(B), 1(C), 2 and 3. In this case, a positioning control system in a magneto-optical disk device will be illustrated representatively.
FIGS. 1(A), 1(B) and 1(C) are diagrams for explaining servo control operations for positioning control in a magneto-optical disk device. To be more specific, FIG. 1(A) is a perspective view schematically showing the whole construction of a magneto-optical disk device; FIG. 1(B) is a front view of each of photo-detectors of two-divisional type; and FIG. 1(C) is a front view of a photo-detector of four-divisional type.
In FIG. 1(A), 10 denotes a magneto-optical disk 10 which functions as an optically recording medium and which is rotated by a spindle motor(not shown). Further, 2 denotes an optical head 2 which irradiates an optical beam to the predetermined position on the magneto-optical disk 10 for recording/reproducing operations. The optical head 2 is adapted to be driven by another motor(not shown) so as to move with respect to the radial direction of the magneto-optical disk 10, and to perform a seek operation for the predetermined track position.
In regard to the configuration on the surface of the magneto-optical disk 10, a plurality of tracks for recording information are formed between adjoining guide grooves. Further, both a tracking position-error and a focusing position-error are obtained by utilizing the reflected optical beam, which is reflected from the magneto-optical disk 10 and goes back to an optical head 2 when an optical beam of the optical head 2 is irradiated to the magneto-optical disk 10.
To be more specific, a tracking position-error signal TES is generated on the basis of the reflected optical beam, and the tracking position-error of the optical beam corresponding to such a tracking servo-error signal TES is output. Further, taking into consideration the tracking position-error, a track control circuit(not shown) is adapted to control the optical head 2 to accurately irradiate the predetermined track for recording/reproducing operations.
In a similar manner, a focusing servo-error signal FES is generated on the basis of the reflected optical beam, and the focusing position-error of the optical beam corresponding to such a focusing servo-error signal FES is output. Further, taking into consideration the focusing position-error, a focusing control circuit(not shown) is adapted to control the optical head 2 so that a focus of the optical beam can accurately conform to the desired track position.
Further, in the optical head 2, the beam emitted from a semiconductor laser 20 is collected by a collimator lens 21. Then, the collected beam is reflected by a reflection mirror 23, through a beam-splitting prism 22 for splitting the beam into a polarized beam, and collected by an objective(also referred to as an actuator) 24, and irradiated to the surface of the magneto-optical disk 10,.
At this time, the magnetizing force is given to the irradiated track of the magneto-optical disk 10 by a magnet 30 which is arranged confronting with the magneto-optical disk 10, and write/read/erase operations on the predetermined track of the magneto-optical disk 10 can be performed.
Further, the reflected optical beam from the magneto-optical disk 10 is collected by the objective 24, and reflected by the beam-splitting prism 22, through the reflection mirror 23. Further, the reflected optical beam from the prism 22 is collected by another collimator lens 26, through a half-wavelength plate 25, and irradiated to both of a tracking photo-detector 29 of two-divisional type and focusing photo-detector 28 of two-divisional type, through another beam-splitting prism 22.
The tracking photo-detector 29 of two-divisional type are constituted by two divided portions so that the moving amount of the optical beam (the hatched portion in FIG. 1(B)) with respect to the radial direction of the track on the surface of the magneto-optical disk 10 can be detected, as shown in FIG. 1(B). Further, in FIGS. 1(A) and 1(B), the respective output terminals of the two divided portions of the tracking photo-detector 29 are connected to a tracking servo-error signal generating circuit 4a. Further, the tracking servo-error signal generating circuit 4a calculates the value of difference between the respective detection levels A, B of the two output terminals of the tracking photo-detector 29, and consequently outputs the tracking servo-error signal TES corresponding to this calculated value of difference.
Further, as shown in FIG. 1(B), the focusing photo-detector 28 of two-divisional type are also constituted by two divided portions. The respective output terminals of the two divided portions of the focusing tracking photo-detector 28 are connected to a focusing servo-error signal generating circuit 4b. Further, the focusing servo-error signal generating circuit 4b calculates the value of difference between the respective detection levels C, D of the two output terminals of the focusing photo-detector 29, and consequently outputs the focusing servo-error signal FES corresponding to this calculated value of difference.
As described above, in the example of FIG. 1(B), two kinds of photo-detectors 28, 29 of two-divisional type are utilized for detecting the tracking position-error and the focusing position-error.
However, as shown in FIG. 1(C), it is also possible for one photo-detector of four-divisional type to be utilized, in place of the two photo-detectors of two-divisional type as in FIG. 1(B). In this case, by means of only one photo-detector of four-divisional type, the tracking servo-error signal TES can be obtained by calculating (A+C)-(B+D), and also the focusing servo-error signal FES is obtained by calculating (A+B)-(C+D).
In such servo-error signal generating circuits as represented by the above-mentioned tracking servo-error signal generating circuit 4a and focusing servo-error signal generating circuit 4b, the respective optical powers of the optical beams utilized in write operations, read operations and erase operations for the magneto-optical disk 10 are usually different from each other. Accordingly, the different photo-current, i.e., different detection current, flows in each of the photo-detectors 28, 29 at every write/read/erase operation, in accordance with the the corresponding optical power of the optical beam. Furthermore, the medium reflection factors of the individual tracks on the magneto-optical disk 10 are also different from each other. Therefore, to obtain assuredly the above-mentioned servo-error signals each adjusted to have a constant amplitude, it is necessary for AGC for the servo-error signals to be executed in the servo-error signal generating circuit.
To perform such an AGC assuredly, the following AGC operation must be usually executed for the detection currents detected by the photo-detectors.
First, two kinds of detection currents from each of the photo-detectors are added together, and then a sum signal is obtained.
Second, one kind of the detection current from each of the photo-detectors is subtracted from another kind of the detection current therefrom, and then a difference signal is obtained.
Third, the above-mentioned sum signal is divided by the above-mentioned difference signal.
Consequently, both of the tracking servo-error signal and focusing servo-error signal, each having a constant amplitude, can be always obtained.
FIG. 2 is a circuit diagram showing a servo-error signal generating circuit of the prior art in the case where a photo-detector of two-divisional type is utilized. In this case, either one of the tracking servo-error signal generating circuit and focusing servo-error signal generating circuit will be illustrated representatively.
In FIG. 2, resistors R1, R2 are connected to the respectively corresponding photo-detector units P1, P2 in the photo-detector of two-divisional type. Further, each detection current is converted to the corresponding voltage(detection voltage), and the thus converted voltage is input to each base of two transistors T5, T6. Further, these voltages are added together by an operational amplifier 11, and a level-shift operation for a sum of these voltages is performed by an operational amplifier 13. Further, the sum of the voltages is input to the common base of two transistors T2, T3 in a multiplier circuit of Gilbert type.
In this multiplier circuit, the other two transistors T1, T4 are provided, the respective bases of which bias voltages of direct current type(also referred to as d.c. bias voltages) VR are applied to, and the respective collectors of which are connected to a power supply of the voltage Vcc, via resistors R5, R6. Further, the respective collectors of the transistors T2, T3 are coupled together and directly connected to the power supply of the voltage Vcc as the common collector. An emitter of the transistor T2 is connected to an emitter of the transistor T1 as the common emitter, while an emitter of the transistor T3 is connected to an emitter of the transistor T4 as the common emitter. Further, the respective emitters of drive transistors T5, T6 are connected to the corresponding constant current source 5a, 5b, via resistors R7, R8.
Between a node of the resistor R7 and the constant current source 5a and another node of the resistor R8 and the constant current source 5b, a resistor R9 is connected. This resistor R9 is adapted to convert the differential voltage X-Y as mentioned below to the corresponding differential current which flows through the drive transistors T5, T6.
In this case, when the detection voltages of the two photo-detector units P1, P2, is respectively X, Y, corresponding to the detection levels A, B, the voltage of the collector of the transistor T1 becomes {(X+Y)-X}/(X+Y), i.e., Y/(X+Y). On the other hand, the voltage of the collector of the transistor T4 becomes {(X+Y)-Y}/(X+Y), i.e., X/(X+Y). Therefore, the difference between the respective collectors of the transistors T1, T4 becomes (X-Y)/(X+Y); Namely, the difference signal corresponding to the value of X-Y is divided by the sum signal corresponding to the value of X+Y. Further, if the difference (X-Y)/(X+Y) is input to an operational amplifier 12 to accurately calculate the difference between the respective collectors of the transistors T1, T4, the servo-error signal(TES or FES), in which the AGC operation as described before has been performed, can be finally obtained.
Here, Ic1 and VBE1 are assumed to be the collector current of the transistor T1 and the voltage between the base and the emitter thereof, respectively. Further, Ic2 and VBE2 are assumed to be the collector current of the transistor T2 and the voltage between the base and the emitter thereof, respectively. Further, Ic5 is assumed to be the collector current of the transistor T5. By these assumptions, the following equations are obtained. EQU Ic1=Is.times.{exp(q*VBE1/kT)-1} EQU Ic2=Is.times.{exp(q*VBE2/kT)-1} EQU Ic5=Ic1+Ic2
Where, q denotes an electric charge of an electron, T denotes an absolute temperature, k denotes a Boltzmann constant, and Is denotes a saturation current of a collector in the reverse direction.
Further, in the case where the condition represented by exp(q*VBE1/kT)&gt;&gt;1 and exp(q*VBE1/kT)&gt;&gt;1 is satisfied, the following equation is obtained. EQU Ic1.apprxeq.Ic5/1+exp{q(VBE2-VBE1)/kT}!
Further, the respective currents detected by the photo-detectors P1, P2 are logarithmically converted by the operational amplifier 11, and then supplied to the common base of the transistors T2, T3 as the base voltage thereof. Therefore, the following equation is further obtained. EQU Ic1=Ic5/{1+K*(X+Y)}
Where, K denotes a proportional constant.
Further, in the case where the voltage VR is selected so that the condition represented by K*(X+Y)&gt;&gt;1 can be satisfied, the following equations are simultaneously obtained. EQU Ic1.apprxeq.Ic5/K*(X+Y) EQU Ic4.apprxeq.Ic6/K*(X+Y)
Where, Ic6 denotes the collector current of the transistor T6.
In this case, since Ic5 and Ic6 correspond to the respective output currents of a differential amplifier having the transistors T5, T6, it is apparent from the above-mentioned equations that Ic1-Ic4 is proportional to (X-Y)/(X+Y).
Further, the collector currents Ic5, Ic6 are converted to the corresponding voltages by the resistors R5, R6, respectively. Thereafter, these voltages are input to the operational amplifier 12. Finally, the differential signal taken out from the operational amplifier 12 can be the tracking servo-error signal TES or the focusing servo-error signal FES, which has a constant amplitude independent of the change of the amount of optical beam.
FIG. 3 is a circuit diagram showing a servo-error signal generating circuit of the prior art in the case where a photo-detector of four-divisional type is utilized. In this case, only the focusing servo-error signal generating circuit will be illustrated as the representative of the servo-error signal generating circuits. Hereinafter, any component that is the same as that mentioned before will be referred to using the same reference number.
In FIG. 3, resistors R41, R42, R43 and R44 are connected to the respectively corresponding photo-detector units P41, P42, P43 and P44 in the photo-detector of four-divisional type. Further, the detection currents of the respective photo-detector units P41, P42, P43 and P44 are respectively converted to the detection voltages A4, B4, C4 and D4, corresponding to the detection levels A, B, C and D illustrated in FIG. 1(C).
The two detection voltages A4, B4 are added together by an operational amplifier 14 including the predetermined external resistors r1, r2 and r3. On the other hand, the remaining two detection voltages C4, D4 are also added together by an operational amplifier 15 including the predetermined external resistors r4, r5 and r6. Two kinds of sums A4+B4 and C4+D4 respectively taken out from the amplifiers 15, 16 are input to the corresponding bases of two transistors T5, T6 in a multiplier circuit having the same circuit configuration as in FIG. 2. Further, the four detection voltages A4, B4, C4 and D4 are added together by an operational amplifier 11 including the predetermined external resistors r7, r8, r9, r10 and r11, and a level-shift operation for a sum A4+B4+C4+D4 of all these voltages is performed by an operational amplifier 13 including the predetermined external resistors r12, r13, r14 and r15. Further, the sum A4+B4+C4+D4 is input to the common base of two transistors T2, T3 in the above-mentioned multiplier circuit.
Further, in a similar manner to the case of FIG. 2, the voltage of the collector of the transistor T1 becomes {(A4+B4+C4+D4)-(A4+B4)}/(A4+B4+C4+D4), i.e., (C4+D4)/(A4+B4+C4+D4). On the other hand, the voltage of the collector of the transistor T4 becomes {(A4+B4+C4+D4)-(C4+D4)}/(A4+B4+C4+D4), i.e., (A4+B4)/(A4+B4+C4+D4). Therefore, the difference between the respective collectors of the transistors T1, T4 becomes {(A4+B4)-(C4+D4)}/(A4+B4+C4+D4); Namely, the difference signal corresponding to the value of (A4+B4)-(C4+D4) is divided by the sum signal corresponding to the value of (A4+B4+C4+D4). Further, if the difference {(A4+B4)-(C4+D4)}/(A4+B4+C4+D4) is input to an operational amplifier 12 to accurately calculate the difference between the respective collectors of the transistors T1, T4, the focusing servo-error signal FES, in which the AGC operation as described before has been performed, can be finally obtained.
Further, if other operational amplifiers and another multiplier circuit, respectively having the same configuration as the above-mentioned amplifiers 12, 14 and 15 and the above-mentioned multiplier circuit of Gilbert type, are provided, it becomes also possible for the tracking servo-error signal TES to be obtained.
However, in the positioning control system including the servo-error signal generating circuit according to the prior art as illustrated in FIG. 2 or FIG. 3, the following problems have occurred.
First, such a servo-error signal generating circuit is mainly constituted by an AGC circuit of the voltage-operation type, in which the detection currents in the photo-detectors have to be converted to the voltage utilized as the servo-error signal. Therefore, it becomes necessary for the circuit to have a lot of resistors and a lot of operational amplifiers in converting the detection currents to the useful voltages. Consequently, the circuit per se is likely to have the complicated configuration, and to be easily affected by external noises.
Second, the circuit has a lot of transistors connected in multi-stage configuration. Therefore, it becomes difficult for the levels of all the transistors to be normally distributed, and also it becomes difficult for the operating points of all the transistors to be accurately determined. Consequently, all the transistors can not be driven by a single power supply.
Third, it is difficult for the circuit including such operational amplifiers and the related resistors to be realized by an IC, which can provide the desired circuit with a small size and in relatively low cost for fabrication.